Radar sensors are used in motor vehicles, for example in an Adaptive Cruise Control (ACC) system, and are used within such a system to measure the distance from a vehicle traveling ahead, so that the speed of the subject vehicle can be adapted to that of the vehicle traveling ahead. A description of such an ACC system can be found, for example, in Winner: “Adaptive Cruise Control” in Automotive Electronics Handbook, Ronald K. Jurgen (editor), 2nd Edition, McGraw-Hill (1999), Chapter 30.1. The principle of distance measurement is based on a measurement of the transit time of the radar echo. The Doppler effect also allows a direct measurement of the relative speed of the vehicle traveling ahead, or of another target object that reflects radar waves. The radar sensor for the ACC system typically operates with electromagnetic waves having a frequency of 77 GHz.
In addition, radar sensors are used in motor vehicles as environmental sensors which also monitor the space to the sides of and behind the subject vehicle. These radar sensors operate at a frequency of 24 GHz, for example. The range of application of the present invention is not limited to a specific region of the electromagnetic spectrum, and may also include the visible light region, for example.
Depending on the embodiment, the transmitter and receiver unit for the radar sensor has either separate antennae for transmitting the radar signal and for receiving the echo, or a single antenna for both transmitting and receiving.
For angular localization of the detected objects, a multibeam radar is often used in which the direction of transmission and reception has multiple transmitting and receiving lobes angularly offset with respect to one another, so that they cover a wider angular range around the straight-ahead direction of the vehicle. Separate evaluation of the radar echoes received by the various receiving lobes makes it possible to determine the direction, i.e., the azimuth angle, of the target object. Since the radar sensors used as distance sensors in motor vehicles are used primarily for detecting vehicles traveling ahead in the same lane and in adjacent lanes, the axes of the multiple transmitting and receiving lobes are situated in a common plane approximately parallel to the roadway surface.
In one conventional embodiment of a multibeam radar sensor, each transmitting and receiving lobe is associated with a unique patch antenna as a transmitter/receiver. The patch antennae are laterally offset with respect to one another in the focal plane of a shared optical lens. The radar waves emitted from each patch are bundled by the lens, which is made of a dielectric material, for example plastic, which is transparent to the radar waves, and are radiated in a direction specified by the position of the patch in the focal plane. During receiving, the same lens is used for refocusing the received echo from the particular receiving lobe onto the associated patch. Depending on the embodiment of the sensor, transmission and reception may be performed at the same time, or with a time delay.